9. Work & Energy
Power
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>> Okay, class, welcome back to another edition of Learning Glass Lectures on Physics. Let's talk about power, and power is one of these terms that you hear all the time, and when you think about power, let me ask you guys what do you think about when you hear that word "power"? Matt, somebody hand a microphone to Matt, and let's have a chat. Matt, what do you think about when you hear the word "power"? Great name, by the way. >> Lightbulbs. >> Lightbulbs, okay. Lightbulbs. Why do you think of lightbulbs? >> I used the most. >> Because when you buy a lightbulb, what does it say on the lightbulb? >> Wattage. >> Wattage, which is power, okay. So, that is important here. The way they classify lightbulbs is in terms of their power rating. What else do you think about when you hear that word "power"? What was your name in front? Yeah. >> Natalia. >> Natalia. Can you hand the mike to Natalia? Natalia, what else do you think about when you hear that word "power"? >> Anything that comes to mind; doesn't have to be related to physics at all. >> I guess, rollercoasters. >> Rollercoasters, okay. What's the power in the rollercoaster? Maybe what's the power in the engine that drives the rollercoaster to the top, how much power is exerted, and so forth? All right. I think that's a good example. And certainly, when you talk about things like cars, right, one of the metrics that they give you, what's the performance of your car, they tell you about the power of the car, all right. That's typically in horsepower, which is literally the power exerted by one standard horse, okay; not, joking. And that can convert to SI units, but in cars they talk about horsepower. So, if you have 200 horsepower in your car, that's like you're in a cart being pulled by 200 horses. That's what it means. It's kind of weird to think about but, you know, people were a little weird back in the day, so. Okay. So, lightbulbs, rollercoasters, cars; anything else that people think about when you hear that word "power"? Would you hand the mike back to Lauren? Let's talk to Lauren for a second. Lauren, what do you think about when you hear that word "power"? >> I kind of think about like kings and queens because they're high in power, I don't know. >> Okay. Royalty. >> Yeah. >> Sure, why not? People that have a lot of power, right? Heads of state, they have a lot of power, right. That's not maybe the physics end of things, but that's certainly a word that gets thrown around there a lot. So, there's a lot of words that mean something in physics that mean something very different out there in the other parts of the world. I would say power with regards to royalty or heads of state is probably one of those, but people misuse these words all the time. For instance, quantum, right, that's one of those words that they like to put on everything. I was reading about this lawn mower the other day that featured the brand new Briggs and Stratton Quantum engine, and I was thinking, do they know that quantum is like really small, that they're bragging about, this has the smallest engine. All right. So, let's talk about power in terms of physics now. What do we know in our everyday lives, and let's relate it back to something that we're all familiar with, your electric bill. Okay. At the end of the month, you get an electric bill from your utility company, and on that electric bill it says not only how much money you owe, but it say something else about the power that you used for that month, and it gives you something called kilowatt hours, right. Kilowatt hours, now, what Matt said was that power is watts. Like when you buy a lightbulb, it says 60 watts on it. So, this thing is apparently power. We know what hours are; that's time. So, what is a kilowatt hour? That's not power, and it's not time; it's power times time. What is that quantity called? Anybody know? I'll give you a hint. When you drink your Red Bull, you say, oh, I suddenly have a lot of -- >> Energy. >> Energy. Energy. That's what your utility bill is. It's not how much power you used; it's how much power you used for how long, and that is the energy that you used. Energy is what your bill is. And so, now, look, we can start to put some stuff together here, right. This is learning physics from your mail. Energy, e, is apparently power times time. So, power is energy over time. Or if all that energy came from some work that you did, then it is work divided by time. Okay. Power is work divided by time. Now, this is one of these cases where it's certainly true if the power is constant, and these things are all constant, then you can just write down this relationship. But in general, we have to be a little bit more careful about it. What we really want to say is work is the integral of power over time, and this takes into account the fact that maybe the power is going up, going down, all right, which happens in your house. You turn on the lights when you get home; you turn them off when you go to bed; you turn the air conditioner on during the day; turn it off at night; so, the power is going up and down all the time. And so the energy that you use, or the work, is going to be the area under the curve. So, if I'm integrating for my month, I have to worry about this entire area, right. This is the area under the curve, and that's really what the utilities measure. They measure that area. They measure that energy that you used for that month. Okay. The SI units of power is the Watt, and we write it with a capital W, and it is apparently work, which we know is joules divided by time, which is second. What's a Watt; it's a joule per second. That's how much energy you use in one second. And it's unfortunate that we're using w's here, and we're also using w's for the units. Sorry about that. Again, we ran out of letters. We tried to go to the Greek alphabet, but we ran out of those too; so, we started to double up on stuff. So, we should just invent some new ones as we go.
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